Network Working Group M. Koster
Internet-Draft SmartThings
Intended status: Standards Track A. Keranen
Expires: September 13, 2017 J. Jimenez
Ericsson
March 12, 2017
Publish-Subscribe Broker for the Constrained Application Protocol (CoAP)
draft-ietf-core-coap-pubsub-01
Abstract
The Constrained Application Protocol (CoAP), and related extensions
are intended to support machine-to-machine communication in systems
where one or more nodes are resource constrained, in particular for
low power wireless sensor networks. This document defines a publish-
subscribe broker for CoAP that extends the capabilities of CoAP for
supporting nodes with long breaks in connectivity and/or up-time.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 13, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. CoAP pubsub Architecture . . . . . . . . . . . . . . . . 4
3.2. CoAP pubsub Broker . . . . . . . . . . . . . . . . . . . 4
3.3. CoAP pubsub Client . . . . . . . . . . . . . . . . . . . 5
3.4. CoAP pubsub Topic . . . . . . . . . . . . . . . . . . . . 5
3.5. Brokerless pubsub . . . . . . . . . . . . . . . . . . . . 5
4. CoAP pubsub API . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. DISCOVERY . . . . . . . . . . . . . . . . . . . . . . . . 6
4.2. CREATE . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.3. PUBLISH . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.4. SUBSCRIBE . . . . . . . . . . . . . . . . . . . . . . . . 13
4.5. UNSUBSCRIBE . . . . . . . . . . . . . . . . . . . . . . . 14
4.6. READ . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.7. REMOVE . . . . . . . . . . . . . . . . . . . . . . . . . 17
5. CoAP pubsub Operation with Resource Directory . . . . . . . . 18
6. Sleep-Wake Operation . . . . . . . . . . . . . . . . . . . . 19
7. Simple Flow Control . . . . . . . . . . . . . . . . . . . . . 19
8. Security Considerations . . . . . . . . . . . . . . . . . . . 20
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
9.1. Resource Type value 'core.ps' . . . . . . . . . . . . . . 21
9.2. Resource Type value 'core.ps.discover' . . . . . . . . . 21
9.3. Response Code value '2.04' . . . . . . . . . . . . . . . 21
9.4. Response Code value '4.29' . . . . . . . . . . . . . . . 21
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
11.1. Normative References . . . . . . . . . . . . . . . . . . 22
11.2. Informative References . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
The Constrained Application Protocol (CoAP) [RFC7252] supports
machine-to-machine communication across networks of constrained
devices. CoAP uses a request/response model where clients make
requests to servers in order to request actions on resources.
Depending on the situation the same device may act either as a server
or a client.
One important class of constrained devices includes devices that are
intended to run for years from a small battery, or by scavenging
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energy from their environment. These devices have limited
reachability because they spend most of their time in a sleeping
state with no network connectivity. Devices may also have limited
reachability due to certain middle-boxes, such as Network Address
Translators (NATs) or firewalls. Such middle-boxes often prevent
connecting to a device from the Internet unless the connection was
initiated by the device.
This document specifies the means for nodes with limited reachability
to communicate using simple extensions to CoAP. The extensions
enable publish-subscribe communication using a broker node that
enables store-and-forward messaging between two or more nodes.
Furthermore the extensions facilitate many-to-many communication
using CoAP.
2. Terminology
The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT',
'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY', and 'OPTIONAL' in this
specification are to be interpreted as described in [RFC2119].
This specification requires readers to be familiar with all the terms
and concepts that are discussed in [RFC5988] and [RFC6690]. Readers
should also be familiar with the terms and concepts discussed in
[RFC7252] and [I-D.ietf-core-resource-directory]. The URI template
format [RFC6570] is used to describe the REST interfaces defined in
this specification.
This specification makes use of the following additional terminology:
Publish-Subscribe (pubsub): A messaging paradigm where messages are
published to a broker and potential receivers can subscribe to the
broker to receive messages. The publishers do not (need to) know
where the message will be eventually sent: the publications and
subscriptions are matched by a broker and publications are
delivered by the broker to subscribed receivers.
CoAP pubsub service: A group of REST resources, as defined in this
document, which together implement the CoAP pubsub service.
CoAP pubsub Broker: A server node capable of receiving messages
(publications) from and sending messages to other nodes, and able
to match subscriptions and publications in order to route messages
to the right destinations. The broker can also temporarily store
publications to satisfy future subscriptions.
CoAP pubsub Client: A CoAP client which is capable of publish or
subscribe operations as defined in this specification.
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Topic: A unique identifier for a particular item being published
and/or subscribed to. A broker uses the topics to match
subscriptions to publications. A topic is a valid CoAP URI as
defined in [RFC7252]
3. Architecture
3.1. CoAP pubsub Architecture
Figure 1 shows the architecture of a CoAP pubsub service. CoAP
pubsub Clients interact with a CoAP pubsub Broker through the CoAP
pubsub API which is hosted by the Broker. State information is
updated between the Clients and the Broker. The CoAP pubsub Broker
performs a store-and-forward of state update representations between
certain CoAP pubsub Clients. Clients Subscribe to topics upon which
representations are Published by other Clients, which are forwarded
by the Broker to the subscribing clients. A CoAP pubsub Broker may
be used as a REST resource proxy, retaining the last published
representation to supply in response to Read requests from Clients.
Clients pubsub Broker
+-------+ |
| CoAP | |
|pubsub |---------|------+
|Client | | | +-------+
+-------+ | +----| CoAP |
| |pubsub |
+-------+ | +----|Broker |
| CoAP | | | +-------+
|pubsub |---------|------+
|Client | |
+-------+ |
Figure 1: CoAP pubsub Architecture
3.2. CoAP pubsub Broker
A CoAP pubsub Broker is a CoAP Server that exposes an API for clients
to use to initiate publish-subscribe interactions. Avoiding the need
for direct reachability between clients, the broker only needs to be
reachable from all clients. The broker also needs to have sufficient
resources (storage, bandwidth, etc.) to host CoAP resource services,
and potentially buffer messages, on behalf of the clients.
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3.3. CoAP pubsub Client
A CoAP pubsub Client interacts with a CoAP pubsub Broker using the
CoAP pubsub API. Clients initiate interactions with a CoAP pubsub
broker. A data source (e.g., sensor clients) can publish state
updates to the broker and data sinks (e.g., actuator clients) can
read from or subscribe to state updates from the broker. Application
clients can make use of both publish and subscribe in order to
exchange state updates with data sources and sinks.
3.4. CoAP pubsub Topic
The clients and broker use topics to identify a particular resource
or object in a publish-subscribe system. Topics are conventionally
formed as a hierarchy, e.g. "/sensors/weather/barometer/pressure" or
"EP-33543/sen/3303/0/5700". The topics are hosted at the broker and
all the clients using the broker share the same namespace for topics.
Every CoAP pubsub topic has a link, consisting of a reference path on
the broker using URI path [RFC3986] construction and link attributes
[RFC6690]. Every topic is associated with zero or more stored
representations with a content-format specified in the link. A CoAP
pubsub topic value may alternatively be a collection of one or more
sub-topics, consisting of links to the sub-topic URIs and indicated
by a link-format content-format.
3.5. Brokerless pubsub
Figure 2 shows an arrangement for using CoAP pubsub in a "brokerless"
configuration between peer nodes. Nodes in a brokerless system may
act as both broker and client. The Broker interface in a brokerless
node may be pre-configured with topics that expose services and
resources. Brokerless peer nodes can be mixed with client and broker
nodes in a system with full interoperability.
Peer pubsub Peer
+-------+ | +-------+
| CoAP | | | CoAP |
|pubsub |---------|---------|pubsub |
|Client | | |Broker |
+-------+ | +-------+
| CoAP | | | CoAP |
|pubsub |---------|---------|pubsub |
|Broker | | |Client |
+-------+ | +-------+
Figure 2: Brokerless pubsub
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4. CoAP pubsub API
This section defines the API exposed by a CoAP pubsub Broker to
pubsub Clients. The examples throughout this section assume the use
of CoAP [RFC7252]. A CoAP pubsub Broker implementing this
specification MUST support the DISCOVERY, CREATE, PUBLISH, SUBSCRIBE,
UNSUBSCRIBE, READ, and REMOVE operations defined in this section.
4.1. DISCOVERY
CoAP pubsub Clients discover CoAP pubsub Brokers by using CoAP Simple
Discovery or through a Resource Directory (RD)
[I-D.ietf-core-resource-directory]. A CoAP pubsub Broker SHOULD
indicate its presence and availability on a network by exposing a
link to its pubsub API at its .well-known/core location [RFC6690]. A
CoAP pubsub broker MAY register its pubsub API location with a
Resource Directory. Figure 3 shows an example of a client
discovering a local pubsub API using CoAP Simple Discovery. A broker
wishing to advertise the CoAP pubsub API for Simple Discovery or
through a Resource Directory MUST use the link relation rt=core.ps.
A broker MAY advertise its supported content formats and other
attributes in the link to its pubsub API.
A CoAP pubsub Broker MAY offer a topic discovery API to enable
Clients to find topics of interest, either by topic name or by link
attributes which may be registered when the topic is created.
Figure 4 shows an example of a client looking for a topic with a
resource type (rt) of "temperature" using Discover. The client then
receives the URI of the resource and its content-format. A pubsub
broker wishing to advertize topic discovery MUST use the relation
rt=core.ps.discover in the link.
A CoAP pubsub Broker MAY expose the Discover interface through the
.well-known/core resource. Links to topics may be exposed at .well-
known/core in addition to links to the pubsub API. Figure 5 shows an
example of topic discovery through .well-known/core.
The DISCOVER interface is specified as follows:
Interaction: Client -> Broker
Method: GET
URI Template: /.well-known/core
URI Template: /{+ps/}{topic}{/topic*}{?q*}
URI Template Variables:
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/.well-known/core := for discovering the pubsub API (optional)
ps := pubsub API path (optional). The base URI path of the
pubsub API, as obtained from discovery, used to discover
topics.
topic := The desired topic to return links for (optional).
q := Query Filter (optional). MAY contain a query filter list
as per [RFC6690] Section 4.1.
Content-Format: application/link-format
The following response codes are defined for this interface:
Success: 2.05 "Content" with an application/link-format payload
containing one or more matching entries for the broker resource.
A pubsub broker SHOULD use the value "/ps/" for the base URI of
the pubsub API wherever possible.
Failure: 4.04 "Not Found" is returned in case no matching entry is
found for a unicast request.
Failure: 4.00 "Bad Request" is returned in case of a malformed
request for a unicast request.
Failure: No error response to a multicast request.
Client Broker
| |
| ------ GET /.well-known/core?rt=core.ps ---->>|
| -- Content-Format: application/link-format ---|
| |
| <;rt=core.ps;rt=core.ps.discover;ct=40 --|
| |
Figure 3: Example of DISCOVER pubsub function
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Client Broker
| |
| ---------- GET /ps/?rt="temperature" ------->>|
| Content-Format: application/link-format |
| |
| <;rt="temperature";ct=50 ---|
| |
Figure 4: Example of DISCOVER topic
Client Broker
| |
| -------- GET /.well-known/core?ct=50 ------->>|
| Content-Format: application/link-format |
| |
| <;rt="temperature";ct=50 ---|
| |
Figure 5: Example of DISCOVER topic
4.2. CREATE
Clients create new topics on the broker using CREATE. A client
wishing to create a topic MUST use CoAP POST to the pubsub API with a
payload indicating the desired topic. The topic specification sent
in the payload MUST use a supported serialization of the CoRE link
format [RFC6690]. The target of the link MUST be a URI formatted
string. The client MUST indicate the desired content format for
publishes to the topic by using the ct (Content Format) link
attribute in the link-format payload. The client MAY indicate the
lifetime of the topic by including the Max-Age option in the CREATE
request.
A Broker MUST return a response code of "2.01 Created" if the topic
is created and return the URI path of the created topic via Location-
Path options. The broker MUST return the appropriate 4.xx response
code indicating the reason for failure if a new topic can not be
created. Broker SHOULD remove topics if the Max-Age of the topic is
exceeded without any publishes to the topic. Broker SHOULD retain a
topic indefinitely if the Max-Age option is elided or is set to zero
upon topic creation. The lifetime of a topic MUST be refreshed upon
create operations with a target of an existing topic.
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Topics may be created as sub-topics of other topics. A client MAY
create a topic with a ct (Content Format) link attribute value which
describes a supported serialization of the CoRE link format [RFC6690]
such as application/link-format (ct=40) or its JSON or CBOR
serializations. If a topic is created which describes a link
serialization, that topic may then have sub-topics created under it
as shown in Figure 7.
The CREATE interface is specified as follows:
Interaction: Client -> Broker
Method: POST
URI Template: /{+ps/}{topic}{/topic*}
URI Template Variables:
ps := pubsub API path (mandatory). The path of the pubsub API,
as obtained from discovery. A pubsub broker SHOULD use the
value "ps" for this variable whenever possible.
Content-Format: application/link-format
Payload: The desired topic to CREATE
The following response codes are defined for this interface:
Success: 2.01 "Created". Successful Creation of the topic
Failure: 4.00 "Bad Request". Malformed request.
Failure: 4.01 "Unauthorized". Authorization failure.
Failure: 4.03 "Forbidden". Topic already exists.
Failure: 4.06 "Not Acceptable". Unsupported content format for
topic.
Figure 6 shows an example of a topic called "topic1" being
successfully created.
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Client Broker
| |
| ---------- POST /ps ";ct=50" -------->|
| |
| ;ct=40" ------->|
| |
| ;ct=50" -->|
| |
|
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requests to a topic while pending resolution of notifications to
subscribers from previous PUT requests.
Create on PUBLISH: A Broker MAY accept PUBLISH operations to new
topics using the PUT method. If a Broker accepts a PUBLISH using PUT
to a topic that does not exist, the Broker MUST create the topic
using the information in the PUT operation. The Broker MUST create a
topic with the URI-Path of the request, including all of the sub-
topics necessary, and create a topic link with the ct attribute set
to the content-format of the payload of the PUT request. If topic is
created, the Broker MUST return the response "2.01 Created" with the
URI of the created topic, including all of the created path segments,
returned via the Location-Path option.
A Broker MAY accept PUBLISH operations using the POST method. If a
broker accepts PUBLISH using POST it MAY store an ordered list of
published representations, with an element of the list for each
published representation. A Broker MAY reject, or delay responses
to, POST requests if the internal capacity to store representations
is exceeded.
A Broker MAY perform garbage collection of stored representations
which have been delivered to all subscribers or which have timed out.
A Broker MAY retain at least one most recently published
representation to return in response to SUBSRCIBE and READ requests.
A Broker MUST make a best-effort attempt to notify all clients
subscribed on a particular topic each time it receives a publish on
that topic. An example is shown in Figure 10. If a client publishes
to a broker with the Max-Age option, the broker MUST include the same
value for the Max-Age option in all notifications. A broker MUST use
CoAP Notification as described in [RFC7641] to notify subscribed
clients.
The PUBLISH interface is specified as follows:
Interaction: Client -> Broker
Method: PUT, POST
URI Template: /{+ps/}{topic}{/topic*}
URI Template Variables:
ps := pubsub API path (mandatory). The path of the pubsub API,
as obtained from discovery.
topic := The desired topic to publish on.
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Content-Format: Any valid CoAP content format
Payload: Representation of the topic value (CoAP resource state
representation) in the indicated content format
The following response codes are defined for this interface:
Success: 2.01 "Created". Successful publish, topic is created
Success: 2.04 "Changed". Successful publish, topic is updated
Failure: 4.00 "Bad Request". Malformed request.
Failure: 4.01 "Unauthorized". Authorization failure.
Failure: 4.04 "Not Found". Topic does not exist.
Failure: 4.29 "Too Many Requests". The client should slow down the
rate of publish messages for this topic (see Section 7).
Figure 8 shows an example of a new value being successfully published
to the topic "topic1". See Figure 10 for an example of a broker
forwarding a message from a publishing client to a subscribed client.
Client Broker
| |
| ---------- PUT /ps/topic1 "1033.3" --------> |
| |
| |
| |
| |
| |
|
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4.4. SUBSCRIBE
CoAP pubsub Clients subscribe to topics on the Broker using CoAP
Observe as described in [RFC7641]. A CoAP pubsub Client wishing to
Subscribe to a topic on a broker MUST use a CoAP GET with Observe
registration. The Broker MAY add the client to a list of observers.
The Broker MUST return a response code of "2.05 Content" along with
the most recently published value if the topic contains a valid value
and the broker can supply the requested content format. The broker
MUST reject Subscribe requests on a topic if the content format of
the request is not supported by the content format the topic was
created with. The broker MAY accept Subscribe requests which specify
content formats that the broker can supply as alternate content
formats to the content format the topic was registered with. If the
topic was published with the Max-Age option, the broker MUST set the
Max-Age option in the valid response to the amount of time remaining
for the value to be valid since the last publish operation on that
topic. The Broker MUST return a response code of "2.04 No Content"
if the Max-Age of the previously stored value has expired. The
Broker MUST return a response code "4.04 Not Found" if the topic does
not exist or has been removed. The Broker MUST return a response
code "4.15 Unsupported Content Format" if it can not return the
requested content format. If a Broker is unable to accept a new
Subscription on a topic, it SHOULD return the appropriate response
code without the Observe option as per as per [RFC7641] Section 4.1.
There is no explicit maximum lifetime of a Subscription, thus a
Broker may remove subscribers at any time. The Broker, upon removing
a Subscriber, will transmit the appropriate response code without the
Observe option, as per [RFC7641] Section 4.2, to the removed
Subscriber.
The SUBSCRIBE interface is specified as follows:
Interaction: Client -> Broker
Method: GET
Options: Observe:0
URI Template: /{+ps/}{topic}{/topic*}
URI Template Variables:
ps := pubsub API path (mandatory). The path of the pubsub API,
as obtained from discovery.
topic := The desired topic to subscribe to.
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The following response codes are defined for this interface:
Success: 2.05 "Content". Successful subscribe, current value
included
Success: 2.04 "No Content". Successful subscribe, value not
included
Failure: 4.00 "Bad Request". Malformed request.
Failure: 4.01 "Unauthorized". Authorization failure.
Failure: 4.04 "Not Found". Topic does not exist.
Failure: 4.15 "Unsupported Content Format". Unsupported content
format.
Figure 10 shows an example of Client2 subscribing to "topic1" and
receiving a response from the broker, with a subsequent notification.
The subscribe response from the broker uses the last stored value
associated with the topic1. The notification from the broker is sent
in response to the publish received from Client1.
Client1 Client2 Broker
| | Subscribe |
| | ----- GET /ps/topic1 Observe:0 Token:XX ----> |
| | |
| | |
| | Notify |
| |
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The UNSUBSCRIBE interface is specified as follows:
Interaction: Client -> Broker
Method: GET
Options: Observe:1
URI Template: /{+ps/}{topic}{/topic*}
URI Template Variables:
ps := pubsub API path (mandatory). The path of the pubsub API,
as obtained from discovery.
topic := The desired topic to unsubscribe from.
The following response codes are defined for this interface:
Success: 2.05 "Content". Successful unsubscribe, current value
included
Success: 2.04 "No Content". Successful unsubscribe, value not
included
Failure: 4.00 "Bad Request". Malformed request.
Failure: 4.01 "Unauthorized". Authorization failure.
Failure: 4.04 "Not Found". Topic does not exist.
Figure 11 shows an example of a client unsubscribe using the
Observe=1 cancellation method.
Client Broker
| |
| ----- GET /ps/topic1 Observe:1 Token:XX ----> |
| |
|
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4.6. READ
A CoAP pubsub client wishing to obtain only the most recent published
value on a topic MAY use the READ interface. For reading, the client
uses the CoAP GET method. The broker MUST accept Read requests on a
topic if the content format of the request matches the content format
the topic was created with. The broker MAY accept Read requests
which specify content formats that the broker can supply as alternate
content formats to the content format the topic was registered with.
The Broker MUST return a response code of "2.05 Content" along with
the most recently published value if the topic contains a valid value
and the broker can supply the requested content format. If the topic
was published with the Max-Age option, the broker MUST set the Max-
Age option in the valid response to the amount of time remaining for
the topic to be valid since the last publish. The Broker MUST return
a response code of "2.04 No Content" if the Max-Age of the previously
stored value has expired. The Broker MUST return a response code
"4.04 Not Found" if the topic does not exist or has been removed.
The Broker MUST return a response code "4.15 Unsupported Content
Format" if the broker can not return the requested content format.
The READ interface is specified as follows:
Interaction: Client -> Broker
Method: GET
URI Template: /{+ps/}{topic}{/topic*}
URI Template Variables:
ps := pubsub API path (mandatory). The path of the pubsub API,
as obtained from discovery.
topic := The desired topic to READ.
The following response codes are defined for this interface:
Success: 2.05 "Content". Successful READ, current value included
Success: 2.04 "No Content". Topic exists, value not included
Failure: 4.00 "Bad Request". Malformed request.
Failure: 4.01 "Unauthorized". Authorization failure.
Failure: 4.04 "Not Found". Topic does not exist.
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Failure: 4.15 "Unsupported Content Format". Unsupported content-
format.
Figure 12 shows an example of a successful READ from topic1, followed
by a Publish on the topic, followed at some time later by a read of
the updated value from the recent Publish.
Client1 Client2 Broker
| | Read |
| | --------------- GET /ps/topic1 -------------> |
| | |
| | |
| | |
| | |
| | Read |
| | --------------- GET /ps/topic1 -------------> |
| | |
| | Broker
Method: DELETE
URI Template: /{+ps/}{topic}{/topic*}
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URI Template Variables:
ps := pubsub API path (mandatory). The path of the pubsub API,
as obtained from discovery.
topic := The desired topic to REMOVE.
Content-Format: None
Response Payload: None
The following response codes are defined for this interface:
Success: 2.02 "Deleted". Successful remove
Failure: 4.00 "Bad Request". Malformed request.
Failure: 4.01 "Unauthorized". Authorization failure.
Failure: 4.04 "Not Found". Topic does not exist.
Figure 13 shows a successful remove of topic1.
Client Broker
| |
| ------------- DELETE /ps/topic1 ------------> |
| |
| |
|
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A CoAP pubsub Broker may alternatively register links to its topics
to a Resource Directory by triggering the RD to retrieve it's links
from .well-known/core. In order to use this method, the links must
first be exposed in the .well-known/core of the pubsub broker. See
Section 4.1 in this document.
The pubsub broker triggers the RD to retrieve its links by sending a
POST with an empty payload to the .well-known/core of the Resource
Directory. The RD server will then retrieve the links from the
.well-known/core of the pubsub broker and incorporate them into the
Resource Directory. See [I-D.ietf-core-resource-directory] for
further details.
6. Sleep-Wake Operation
CoAP pubsub provides a way for client nodes to sleep between
operations, conserving energy during idle periods. This is made
possible by shifting the server role to the broker, allowing the
broker to be always-on and respond to requests from other clients
while a particular client is sleeping.
For example, the broker will retain the last state update received
from a sleeping client, in order to supply the most recent state
update to other clients in response to read and subscribe operations.
Likewise, the broker will retain the last state update received on
the topic such that a sleeping client, upon waking, can perform a
read operation to the broker to update its own state from the most
recent system state update.
7. Simple Flow Control
Since the broker node has to potentially send a large amount of
notification messages for each publish message and it may be serving
a large amount of subscribers and publishers simultaneously, the
broker may become overwhelmed if it receives many publish messages to
popular topics in a short period of time.
If the broker is unable to serve a certain client that is sending
publish messages too fast, the broker MUST respond with Response Code
4.29, "Too Many Requests". This Response Code is like HTTP 429 "Too
Many Requests" but uses the Max-Age Option in place of the "Retry-
After" header field to indicate the number of seconds after which to
retry. The broker MAY stop creating notifications from the publish
messages from this client and to this topic for the indicated time.
If a client receives the 4.29 Response Code from the broker for a
publish message to a topic, it MUST NOT send new publish messages to
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the broker on the same topic before the time indicated in Max-Age has
passed.
8. Security Considerations
CoAP pubsub re-uses CoAP [RFC7252], CoRE Resource Directory
[I-D.ietf-core-resource-directory], and Web Linking [RFC5988] and
therefore the security considerations of those documents also apply
to this specification. Additionally, a CoAP pubsub broker and the
clients SHOULD authenticate each other and enforce access control
policies. A malicious client could subscribe to data it is not
authorized to or mount a denial of service attack against the broker
by publishing a large number of resources. The authentication can be
performed using the already standardized DTLS offered mechanisms,
such as certificates. DTLS also allows communication security to be
established to ensure integrity and confidentiality protection of the
data exchanged between these relevant parties. Provisioning the
necessary credentials, trust anchors and authorization policies is
non-trivial and subject of ongoing work.
The use of a CoAP pubsub broker introduces challenges for the use of
end-to-end security between for example a client device on a sensor
network and a client application running in a cloud-based server
infrastructure since brokers terminate the exchange. While running
separate DTLS sessions from the client device to the broker and from
broker to client application protects confidentially on those paths,
the client device does not know whether the commands coming from the
broker are actually coming from the client application. Similarly, a
client application requesting data does not know whether the data
originated on the client device. For scenarios where end-to-end
security is desirable the use of application layer security is
unavoidable. Application layer security would then provide a
guarantee to the client device that any request originated at the
client application. Similarly, integrity protected sensor data from
a client device will also provide guarantee to the client application
that the data originated on the client device itself. The protected
data can also be verified by the intermediate broker ensuring that it
stores/caches correct request/response and no malicious messages/
requests are accepted. The broker would still be able to perform
aggregation of data/requests collected.
Depending on the level of trust users and system designers place in
the CoAP pubsub broker, the use of end-to-end object security is
RECOMMENDED [I-D.selander-ace-object-security].
When only end-to-end encryption is necessary and the CoAP Broker is
trusted, Payload Only Protection (Mode:PAYL) could be used. The
Publisher would wrap only the payload before sending it to the broker
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and set the option Content-Format to application/smpayl. Upon
receival, the Broker can read the unencrypted CoAP header to forward
it to the subscribers.
9. IANA Considerations
This document registers one attribute value in the Resource Type
(rt=) registry established with [RFC6690] and appends to the
definition of one CoAP Response Code in the CoRE Parameters Registry.
9.1. Resource Type value 'core.ps'
o Attribute Value: core.ps
o Description: Section 4 of [[This document]]
o Reference: [[This document]]
o Notes: None
9.2. Resource Type value 'core.ps.discover'
o Attribute Value: core.ps.discover
o Description: Section 4 of [[This document]]
o Reference: [[This document]]
o Notes: None
9.3. Response Code value '2.04'
o Response Code: 2.04
o Description: Add No Content response to GET to the existing
definition of the 2.04 response code.
o Reference: [[This document]]
o Notes: None
9.4. Response Code value '4.29'
o Response Code: 4.29
o Description: This error code is used by a server to indicate that
a client is making too many requests on a resource.
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o Reference: [[This document]]
o Notes: None
10. Acknowledgements
The authors would like to thank Hannes Tschofenig, Zach Shelby, Mohit
Sethi, Peter van der Stok, Tim Kellogg, Anders Eriksson, Goran
Selander, Mikko Majanen, and Olaf Bergmann for their contributions
and reviews.
11. References
11.1. Normative References
[I-D.ietf-core-resource-directory]
Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE
Resource Directory", draft-ietf-core-resource-directory-09
(work in progress), October 2016.
[I-D.selander-ace-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security of CoAP (OSCOAP)", draft-selander-ace-
object-security-06 (work in progress), October 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
.
[RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570,
DOI 10.17487/RFC6570, March 2012,
.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
.
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[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
.
11.2. Informative References
[RFC5988] Nottingham, M., "Web Linking", RFC 5988,
DOI 10.17487/RFC5988, October 2010,
.
Authors' Addresses
Michael Koster
SmartThings
Email: Michael.Koster@smartthings.com
Ari Keranen
Ericsson
Email: ari.keranen@ericsson.com
Jaime Jimenez
Ericsson
Email: jaime.jimenez@ericsson.com
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